This invention relates to microwave devices, and in particular to high isolation micromechanical microwave switches.
Microwave switches are used in many telecommunications applications, in particular satellites. Such switches should have high isolation, low insertion loss, and low return loss. Typically, the performance of a switch degrades linearly with increasing frequency.
An example of a typical microwave switch is described in U.S. Pat. No. 5,578,976, the contents of which are herein incorporated by reference. In this device, a micro mechanical cantilever arm causes a contact to close a gap in the signal line when the switch is in the on position. The cantilever arm is actuated by electrostatic forces. While the device described in U.S. Pat. No. 5,578,976 offers an improvement in isolation, it is still limited to about xe2x88x9250 dB at 4 GHz.
An object of the invention is to provide an improved microwave switch.
According to the present invention there is provided a micro-mechanical microwave switch comprising a signal line formed o a substrate and determining a gap forming an open circuit in the off-state of the switch; a dielectric displaceable support member carrying a contact to bridge said gap and close said switch in the on-state; and at least one shield electrode in the vicinity of said gap to reduce the coupling across said gap in the off-state of the switch. The support member is typically a cantilevered arm, although other arrangements, such as a displaceable membrane, can be employed.
In accordance with the principles of the invention, a shield electrode can be placed on the support member for the contact. The shunt capacitance created by the shield electrode significantly reduces the coupling across the gap. A simple shield electrode of this nature will reduce the coupling by as much as 6 dB.
In addition, or alternatively, a shield electrode can be placed under the gap, preferably as a buried layer in the supporting substrate. This serves to re-distribute the electromagnetic field in a manner that also reduces the coupling across the gap. In a preferred embodiment, a shield electrode is fixed above the movable switch contact. In the raised position of the contact, the shield electrode abuts the support member. The shield electrode in this case can be formed on a second substrate bonded to the first.
The invention also provides a method of improving the isolation of a micro-mechanical microwave switch, wherein a signal line formed on a substrate defines a gap forming an open circuit in the off-state of the switch and a displaceable support member carries a contact to bridge said gap and close said switch in the on-state, comprising the step of providing a shield electrode in the vicinity of said gap to reduce the coupling across the gap.